1. Field of the Invention
The present invention relates to a digital payment transaction system.
2. Related Art
With the growth and commercialisation of the internet, there has been an increasing need for technologies to allow payments to be made on-line. For transactions of relatively high financial value this need is adequately met, for example, by systems using electronic cheques issued by a trusted party such as a bank. Such electronic cheques are typically validated by a signature which is encrypted using a public key algorithm such as RSA. There is however a significant computational overhead associated with the use of such algorithms. Therefore, just as in real life a cheque is unlikely to be acceptable for a purchase of small value because of the associated transaction costs, so also in electronic commerce, electronic cheques are not suitable for payments of low value.
A number of proposals have been made for alternative transaction systems suitable for making the so-called "micropayments" required by low-value transactions. However, it has proved technically difficult to provide the low processing overheads required for any micropayment technology whilst maintaining an adequate level of security.
One example of a previous proposal for a micropayment system is that developed by the US corporation Digital and known as "Millicent". This system is described by its proponents as a lightweight protocol suitable for supporting purchases costing less than a cent. It is based on decentralised validation of electronic cash at the vendor's server. The digital payment tokens in this system are termed "scrip". They are issued by a central payment service in return for prepayment using a conventional payment method such as a credit card. The vendor may then accept scrip from the user in payment for goods or more typically for services. The vendor generates fresh scrip and returns it to the user as change for the transaction. The scrip is authenticated and fresh scrip generated by the vendor using a hash function. This represents a potential security weakness, in that if the hash function is cracked, then the scrip would be open to forgery and duplication. Moreover, there is a processing overhead associated with the use of the hash function by the vendor. Although this is less than the overhead associated, for example, with the use of a PGP-encrypted signature, it is nonetheless a significant limitation. It is admitted by the proponents of the Millicent system that, as a result of its limited efficiency, there is a practical lower bound to the transaction values it can handle. It is suggested that this lower bound is around 1/10 of a cent. Millicent is therefore not suitable for use, for example, as a charging mechanism for internet usage. The costs of packet transmission on the internet have been estimated at around 1/600 th of a cent.
European patent application EP-A-0507669 discloses an example of another type of payment system, based on smart card technology. Here, rather than cryptographic security being relied upon, security is based on the physical integrity of the card. A randomly selected sub-set of a number of token values is withheld, so that the presence of one of the withheld token values in a subsequent transaction can serve as an indication of attempted fraud. The set of tokens issued to a particular card is not statistically random but may, for example, all fall within a limited range of numerical values, and may be ordered in sequence determined by their numerical values.